In digital communications in general and in wireless communications in particular, the transmitted data is disturbed by a number of different imperfections. For example, when the signal is transmitted over a channel, it might be interfered with by other signals, or it might be distorted because the transmission channel is time-dispersive. Also, the signal will usually be significantly attenuated by the transition channel. In addition to the disturbances caused by the channel, the transmitter and the receiver will also distort the transmitted signal. The major non-idealities in transmitters are caused by phase noise and non-linearity in the power amplifier of the transmitter. For the receiver, the major imperfections to consider are non-linearities in the receiver front-end and phase noise. The phase-noise, both in the transmitter and the receiver, is primarily caused by the jitter in the frequency synthesizer of the carrier signal.
In order to limit the effect of non-linearities in the transmitter as well as in the receiver, it is advantageous to use signals with constant envelopes. This is for instance the case in GSM and in Bluetooth, see for example, J. C. Haartsen, “The Bluetooth radio system,” IEEE Personal Communications, vol. 7, No. 1, February 2000. In constant amplitude signals no information is transmitted in the amplitude, and all information is in the phase of the transmitted signal. A severe impairment for systems using this type of modulation is therefore the above mentioned phase noise. Since the phase noise is generated at both the transmitter and the receiver, its negative effect cannot be eliminated merely by making the received signal stronger. Often, problems with phase noise are counteracted by designing the system such that the effect of the phase noise will be negligible. This can, e.g., be achieved by the use of powerful error correcting coding. In many cases, the error correcting code is required anyway, and the effects of phase noise will not be an issue.
However, there are situations when it is desirable not to use error correcting coding. One such example is Bluetooth, where uncoded transmission is used to achieve the highest throughput. In fact, for systems operating in the unlicensed ISM band at 2.4 GHz, error correcting coding is usually not very effective. Instead, it is often preferable to use only error detecting coding since error correcting coding is not efficient if the channel conditions change greatly during transmission. That is, either the channel is so good that error correcting coding is not required, or the channel is so poor that the error correcting code is simply not powerful enough to correct the errors present in the transmitted signal. These two cases typically correspond to a situation in which an interfering signal is absent and present, respectively.
Since in many situations it is not desirable to use error correcting coding merely to deal with phase noise, it would be desirable to be able to use an uncoded transmission that still does not suffer too much from phase noise. A simple way to achieve this result is to use differential demodulation in which the receiver will first differentiate the signal before extracting the information. Since the phase noise typically is a relatively slow process because its bandwidth is significantly smaller than the symbol rate, such a differentiation significantly reduces the negative effects of phase noise. However, it is well known that a non-coherent differential receiver suffers in sensitivity compared to a coherent one, and an even greater disadvantage of a non-coherent differential receiver is that it is much more sensitive to channel imperfections such as time-dispersion. Thus, there is a tradeoff in the ability to handle phase noise and the ability to handle time-dispersion effects. Specifically, one can either choose to use a non-coherent receiver to counteract the phase noise, or one can choose a coherent receiver to obtain good performance in time-dispersive channels. Consequently, if one has to counteract the phase noise, then the performance in time-dispersive channels will be poor.
In accordance with at least one embodiment of the present invention, a method and an apparatus is provided in which the above mentioned trade-off is avoided. At least one other embodiment of the present invention provides for the use of a coherent receiver, or close to coherent receiver, in which the effect of phase noise does not have a devastating impact on performance.